Grid electrode structure and manufacturing method therefor



Apni 17, 1951 E. J. WALSH 2,549,551

GRID ELECTRODE STRUCTURE AND mmumcmmc v METHOD mam-"oa- Filed Jan. 15,1948 INVENTOR E. J. WALSH ATTORNEY the electrodes. conductance, it isnecessary to reduce the di- Patented Apr. 17, 1951 'GRID ELECTRODESTRUCTURE AND MANU- FACTURING METHOD THEREFOR Edward J. Walsh, Tenafly,J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application J anuary15, 1948, Serial No.2,433

1 This invention relates to electronic discharge devices and moreparticularly to such devicesof the micro-midget multi-element typeespecially useful in ultra-high frequency translating systems.

The static characteristics of electronic discharge devices determine thescope of use and the efficiency of operation in low frequencyapplicatio-ns' However, 7 the requirements encountered in the very highand ultra-high frequency ranges, such as the 100 to 500-megacycle range,impose conditions which can only be satisfied by extremelyprecisemechanical corelation of the elements to produce the essentialelectrical constants whereby the highest efficiency is attained. Themost important characteristic for general use is the transco-nductance.The transconductance depends on the spacing between the cathode, controlgrid, and screen grid, the diameter of the grid wire, the spacingthereof, the electrode area and the potentials applied to In order toattain a high transameter of the control-grid lateral wire to a pointwhere the wire is not self-supporting when wound in grid form.Therefore, the fine fragile wire must be wound on a support frame whichmaintains the collateral relation of grid and cathode constant at veryclose spacing, for example at of the order of 2 mils. In addition, thegrid laterals must be relatively close together along the length of thegrid to secure the desired controlling action upon the electron streamemanating from the cathode. V

The attainment of relatively close precision relationship between thecathode surface and the control electrode and the closely spaced lateralturns of, the fragile wireof the control electrode practically compelsthe employment of a sheet metal of highly refractory material as theframe or foundation support for the wire, .to

tion, sufficient rigidity to withstand tensioning of the fine wireduring winding, and the maintenance of the laterals in accuraterelatio-nwithout complicated fasteningmeans on the frame structure.

An object of this invention is to facilitate the fabrication of thecontrol electrode assembly in micro-midget electron discharge devicesespecially suitable for use at very high and ultra-high frequencies.

Another, object of the invention'isto maintain ,constant relationbetween the winding of the con- 8 Claims. (01. 250-275) 2 trol electrodeand the cooperating cathode surface of the device.

A further object of the invention is to insure the rigidity of theclosely spaced lateral windings on a frame type grid structure in thedevice.

Another object of the invention is to provide an efiicacious'method ofassembly of the control electrode whereby the lateral turns of wire onthe frame are definitely secured to the frame structure.

A further object of the invention is to enable fabrication of the.control electrode in an expeditious manner to eliminate distortion ofthe fine wire and efficiently bind .the .wire on the frame assembly.

- These objects arerealized, in accordance with features of thisinvention, by the construction of a control electrode of precisedimensions in which relatively fine wire is wound on a hollow metallicframe of lattice configuration having lateral half sections ofrectangular form 'with arcuate uprights which are joined collaterallyalong their the complete assembly. The two half sections of the framestructure are matched to form a hollow support with the arcuate uprightsin abutting relation and the bracing straps in parallel positionstransverse to the uprights. The fine wire is wound continuously aroundthe hollow frame over the longitudinal expanse betweenthe insureadequate mounting, freedom from distorstraps and also over the strapsuntil the Winding is completed. The end turns of wire are temporarilytacked on the frame by affixing a soluble cement over the straps toprevent unwinding of the helical turns of closely spaced wire; After thewinding is completed, a fusible metal advantageous'ly in the form of afew turns of gold wire,

is applied to diagonally opposed ends of the uprights and the wholeassembly is placed in a furnace or oven and'heated in a reducingatmosphere to a sufficient temperature to fuse the metal on the ends ofthe uprights.

During the heating cycle, the fusible metalis reduced to a fluid stateand flows by capillary action along the conjoint edges of the uprightsto braze the helical turns of wire to the frame and also the halfsections of the frame together whereby the helix of wire is rigidlyaffixed to the frame structure. As the heating progresses, the temporarycement binder is removed by evaporation and the fused metal forms apermanent binder for the turns of Wire on the frame. After the structurehas cooled to room temperature in the oven, to avoid oxide contaminationthe loose extremities of the windings on the bracing straps may beremoved.

This arrangement provides an efiicient assembly wherein the helical gridwinding is rigidly fixed on the frame structure and the two sections ofthe frame are joined contiguously to form a hollow enclosure of precisedimensions with respect to the major and minor axes so that the requireddimensional spacing of the grid laterals with respect to the cathodesurface is maintained constant. Furthermore, the heat treatment incidentto the melting of the 'metal is not detrimental to the tension stress ofthe fine grid wire or the frame parts since" these components are highlyrefractory metals so that the initial tension of the wire whichmaintains the laterals in close parallel relation is not destroyed oraffected.

These and other features and advantages of the invention will be moreclearly set forth in the following detailed description of oneembodiment of the invention disclosed in the accompanying drawings inwhich:

Fig. 1 is an elevational view partly in section of a multielementdischarge device embodying features of this invention, with the greaterportion of the vessel and the internal electrodes broken away to clearlyshow the position of the control electrode assembly;

Fig. 2 is a cross-section plan view of the device of Fig. 1 taken on theline 2-2;

Fig. 3 is an enlarged perspective view of the frame structure of thecontrol electrode without the helical winding thereon to show thedetailed structure of the frame and elemental parts thereof;

Fig. 4 illustrates in an elevation view the controlgrid assembly afterthe winding is applied but prior to the final processing and shows thepreliminary binder affixing the fine wire to the frame and the positionof the brazing material on the uprights of the frame assembly; and

Fig. 5 shows a grossly enlarged portion of the grid assembly in across-sectional view taken on the line 5-5 of Fig. 4 and therelationship of the winding and frame in their final integrated formready to be inserted in the unitary electrode assembly of Fig. 1.

Referring to the drawing, the electronic discharge device ofmicro-midget dimensions embodying this invention is shown in Fig. 1 toan enlarged scale of about 4:1 to clearly illustrate the components ofthe unitary electrode assembly mounted within the receptacle. Althoughthe electrode unit, exclusive of mounting projections, occupies lessthan half'of the space in the receptacle, and the mounting includesspacer discs to fit the inner wall of the receptacle, some realizationof the microdimensions of the electrode unit may be gained by reducingthe dimensions'shown by a factor of 4. Then the unit has dimensionallimits of approximately onehalf inch length, one-half inch width andonequarter inch thickness. Since the unit includes a multiplicit ofindividual coaxial electrodes of which the central cathode andsurrounding control electrode must be proportionally smaller in twodimensions, namely, width and thickness, it

is evident that extremely precise tolerances must be maintained in thefabrication of these elements to realize the close spacial relation inthe unitary assembly.

For example, the control grid structure, to which this inventionparticularly appertains, has a width of .228 inch iDOl inch, in aparticular embodiment, a length of .333 inch *-.010 inch and an over-alltransverse dimension of .0490 inch 10006 inch with the minor axis notgreater than .0330 inch :0006 inch, to attain the uniform collateralrelationship of the fine wire laterals supported on the controlelectrode with respect to the cathode surface of not greater than .0025inch. Since the fine tungsten wire supported on the control grid mayhave a diameter within the range of .00022 inch to .00032 inch and theturns per inch may vary between 384 to 326 respectively, for a givenamplification factor, it is evident that precision control must beexercised throughout the fabrication of the elements to attain therequired dimensional geometry of the elements and satisfy the electricalcharacteristics desired in the device.

The device, as shown in Fig. 1, includes an evacuated vitreous enclosingvessel l0, of cylindrical configuration having a molded or cast stem Hsealed to the base end with a plurality of terminal pins I2 projectingfrom the stem for connection to the electrodes in the unit assembly ormount within the vessel. Since this invention is directed to theconstruction of the control grid assembly, its method of assembly andspacial relationshipwith the cathode surface ln the unit, thedescription with respect to the electrode. unit will be general insubstance. A more detailed description will be found in Patent2,507,706, issued May 16, 1950, to R. C. Gee and W. Gronros.

The unit or mount includes a pair of spacer insulating discs l3 and I4,advantageously of mica, which support the electrodes in fixed positionin the enclosing vessel. These discs are attached to opposite ends of aplurality of electrodes including a central rectangular hollow cathodel5 containing an internal heater element IS, a wire wound screen grid I!having parallel support posts If}, a wire Wound suppressor grid l9 and asheet metal formed anode 20 surrounding the inner electrodes. Inaddition suitable metallic shields 2i and 22 are mounted on oppositeends of the unit and attached to the spacer discs, to reduce thecapacitance between the control grid 24 and the anode 20. A gettermounting 23 is supported from the top of the unit. Suitable connectionsextend between the respective electrodes and the terminal pins l2 tocouple the individual electrodes in desired arrangements in electricalcircuits or functional applications to .convert, amplify, control ormodulate the signal voltages intended to be applied in the operation ofthe device.

A precision fabricated control electrode or grid 24 is -mounted in closespace relationship to the cathode I5 and comprises a fine wire helix,for example of tungsten. Since the very fine tungsten wire which forms acomponent of the control grid 24 is not sufficientlystrong to beself-supporting like the laterals of the other grids in the device, asubstantially rigid frame structure is provided to form the foundationor support for the fine wire. This frame structure is clearly shown onan enlarged scale of about 10:1 in Fig. 3 and is formed of a highlyrefractory metal, such asmolybdenum, to withstand the tension c'icecwti5. strainimpo'sed on thefine wire during the winding of the grid. Thegrid" frame preferably is fabricated from sheet metal although it may beformed of tubular sac-Kira single piece construction is desired. Forpracticalr'easons, it is convenient to form the frame support of'twosimilar portions or sections of punched sheet metal of .008'inchthickness, each having a pair of side or upright parallel-struts25'and26 which are joined by transverse bracing straps or bridging arms '21and 28 adjacent opposite ends of the strut portions to form arectangular frame. Each strut'is formed into arcuate shape toward theouter periphery so that when both section's are'placed in abuttingrelation to forma hollow body, the combined struts aresubstantiallysemicircular in cross-section with their contacting edgesin'abutment along the longitudinal median plane of the'body,as'sho'wn'in Fig. 3; One of the struts, for example '26, may be.elongated, as

shown at 29, to provide anextension for attach ing a connecting wirethereto, as shown in the assembly of Fig. 1. 'By forming the framestructure of rigid metal of highly refractory char- 'acter, thedimensions of the major and minor .This dimensional difference iSIIllOleclearly illustrated in Fig. 5 which shows a portionof the grid framestructure in greatly enlarged proportions of 25:1.

After the frame structure is formed, as heretofore described, with thetwo sections which comprise the unit frame of the grid in longitudinalabutting relation to, form a hollow body, the frame is ready for thewinding of the fine wire in helical form with the laterals closelyspaced in parallel relation along the major-portion-of the framestructure. The frame sectionsare placed in pairs on a suitable windingjig, in'series to form a gang ofgrids so that the winding may beperformed as a continuous operation. The winding of the wire is uniformon the frame-over the area between the arms 21'and28 and variableoverthe intermediateportions of adjacent grids, 'to reduce lossof wirein the winding-operation. Since the fine tungsten wire employed inwinding the grid is of very small diameter, of the order of -,00022 inchto .00032 inch; and'the winding pitch is relatively fine, for example326 to 384turns per .inch, and the :grid may even ;be wound with smallerdiameter wire at therateofBOOto '600 turns per inch, it is importantthatprecisionbe exercised in the winding operation to avoid brealy age ofthe wire, due to tensioningand'to insure the close precision in thespace zrelation of, the laterals on the frame structure, it isconvenient the uniformly sp'a'ced laterals must be held in accuratealignment until permanently affixed to the frame. This is achieved byapplying-"the re- 'quired tension to the wire during the windingoperation so'that the uniform pitch'of-the turns is s'tableon the frame.This requires a strong metal frame to withstand the tension strainapplied during the winding so that distortion of the miniature framedoes not occur. "Molybdenum "or other highly refractory metal isparticularly suitable for this purpose. Another advantage of theconstruction involving the metal frame and fine wire with respect toholding the laterals under constant-tension during high temperatureoperation is the fact that the molybdenum frame hasa higher coefficientthan tungsten whereby the'lateral turns of wire on the frame are'heldtaut by the greater expansion of 'the frame in relation to the lowerexpansion of the tungsten wire.

A suitable methodof tensioning the fine wire is disclosed in theapplication Serial No. 775,733, filed'September 23, 1947, of J A.Morton.

fig. 4, which is drawn to the same scale as the grid frame in'Fig. 3,shows the fine wire helix 3| wound at a uniform pitch of approximately360 turns per inch on the major portion of the frame structure betweenthe bridging arms 21 and 28. Since the winding, as heretofore explained,is continuous over a gang of grids in series, the fine wire is woundover the projections of the struts 25 and 26 on adjacent frames in Wideturns and as the Wire nears the bridge arm 21 or 28, the

pitch is'reduced to substantially the required uniform pitch desired onthe main portion of the frame. However, the instant change from the widepitch to the desired pitch is not practical so that as the windingprogresses near the bridging arm, the pitch is gradually reduced so thatthe pitch of the initial turns 32 embracing the struts and arms isvariable for a short distance. The excess turns of the helical wirewinding are located on the bridging straps where they do not enter intothe functional operation of the grid assembly.

When the grid helix (H is completely wound on the frame in the desiredpitch to .conform to the characteristics of the device in which the gridforms one of the components, the variable pitch turns 32 of the wire incontact with the arms 21 and 28 are fixed thereto, preferably by a spotof reducible cement 33, such as nitrocellulose 'cement, which tacks theends of the Winding on the respective arms of each grid assembly so thatthe individual grids may be separated after the Winding operation iscompleted.

The partially complete grid assembly is removed from thewinding jig,after severing the stretch wire between adjacent. grid frames, and

.the individual frame structures are prepared for the final operation topermanently aifix the fine wire on the frame structure. Sinc the framesections are intimately tied together by the surrounding helix ofwireextending along the greater length of the structure, thegridassembly may be conveniently handled after removal from the winding jig.The next step is to apply a low melt- .ing point metal to oppositelydisposed ends of "to perform the winding operation on a precision,.

grid .windingmachine under aipowerfulmi'croscope, to permit the operator'to accurately control the winding operation. Since the fine wire is 1wound continuously over the frame sections,

the strut portions 25 and 26, for example, by winding a few turns of.003 inch gold wire or similar low fusing noble metal wire 34, aroundthe projecting portions ofthe struts relatively close to the junction ofthe bridging arms to the struts. If desired the-frame sections'maybepreviously gold plated and the wire 3| similarly plated before thewinding operation.

When the grid structure is prepared as above described, the wound gridis placed in an electically heated oven while hydrogen is flowingthrough the oven and the grid structure is raised to a temperature ofapproximately 1070 C. for to 30 seconds. This heating step fuses thegold wire 34 which flows by capillary action along the longitudinaljoints between the abutting sections of the frame whereby the joints arebrazed by a filling of gold 35, as shown in Fig. 5, and the turnsof wire3| are ailixed to the arcuate contour of the struts and 26 to rigidlyhold the adjacent turns in their uniform pitched relation. The fluidgold metal also flows out onto the lateral wires during the brazingoperation. Thisis an important feature since a gold surface on the wiresis desirable to prevent excessive primary emission. During the fusing ofthe Wire to the frame structure, the cement 33 will be evaporated butsince the brazing metal 35 intimately embeds the turns of wire 3! to thestruts there is no necessity to retain the temporary binder 33 on theends of the helix applied to the frame. It will be noted from Fig. 5that the inner surface of the bridging arm is in the same plane as theouter flat surface of the strut so that the arm extends outwardly fromthe inner plane of the Wire helix equivalent to the thickness of thearm. Since there will be a few loose turns of wire extending across theends of the grid structure over the area of the arms 21 and 28, theseexcess turns of Wire may be removed so that only the uniform pitchwinding over the area of the grid between the arms will remain on theframe assembly.

The grid assembly of this invention constitutes a unitary ruggedstructure in which the frame sections may be accurately proportioned tosatisfy precision dimensional limits as to the major and minor axes ofthe grid so that the grid assembly may be cooperatively mounted inrelation to other electrodes in the device to secure the requiredelectrical characteristics. The construction also facilitates thefabrication of the grid assembly whereby the fine wire helix is joinedto the frame and the frame sections simultaneously brazed together toprovide an integrated structure. This result is attained in anexpeditious manner by the capillary flow method of the embedding metalwhich quickly locks the wire on the frame and prevents loss of tensionin the wire, which would result in sagging of the turns of wire on theframe. a

While the invention has been disclosed in a particular embodiment andmethod of operation, it is, of course, understood that variousmodifications may be made in the materials, shape of grid frame, windingmethod and processing to achieve the results of this invention and suchmodifications are intended to be Within the scope of the appendedclaims.

What is claimed is:

1. A frame type grid electrode comprising a pair of upright portions inparallel relation and parallel pairs of bracing 'straps extendingbetween said uprights adjacent opposite ends thereof, and a helicalwindingof fine wire embracing said uprights and extending over the areabetween said straps, said Winding having the lateral turns uniformlyspaced in parallel relation over the longitudinal area between saidstraps, said turns being held on said uprights by fusible metalembedding the portions of said laterals on said upright portions.

2. A hollow frame type grid electrode comprise ing a pair of uprightportions of semicircular cross-section in parallel relation and parallelpairs of bracing straps joined to said uprights adjacent opposite endsand extending transversely thereto, said straps being spaced apart adistance greater than the internal dimensions of the open edges of saiduprights, and a helical winding of fine Wire embracing said uprights andextending over the area between said straps, said winding having thelateral turns uniformly spaced in parallel relation over thelongitudinal area between said straps, said turns being held on saiduprights by a fusible metal embedding the portions of said turns on thecurved parts of said upright portions.

3. A hollow frame type grid electrode comprising a pair ofuprightportions of semicircular cross-section in parallel relation and parallelpairs of bracing straps integrally joined to said uprights adjacentopposite ends and extending transversely thereto, the inner surfaces ofsaid straps being in alignment with the outer surfaces of said uprights,and a helical winding of fine wire embracing said uprights and extendingover the area between said straps, said winding having the lateral turnsuniformly spaced in parallel relation over the longitudinal area in linewith the outer surfaces of said uprights and the inner surfaces of saidstraps, said turns being held on said uprights by a fusible metalembedding the portions of said laterals on said upright portions.

4. A hollow frame type grid electrode comprising a pair of half framesections having arcuate longitudinal upright portions and spacedparallel bracing straps adjacent opposite ends of said portions, saidupright portions being placed in abutting relation and formingsemicircular pillars coupled together by said bracing straps, and ahelical Wire Winding embracing said upright portions, the turns thereofbeing held in equally spaced relation by fused metal between saidstraps.

5. A hollow frame type grid electrode comprising a pair of half framesections having arcuate longitudinal upright portions, and spacedparallel bracing straps adjacent opposite ends of said portions, saidupright portions when placed in abutting relation forming semicircularpillars coupled together by said bracing straps, and a helical wirewinding embracing said upright portions, said winding lying in parallelplanes coincident with the inner surfaces of said straps and the outersurfaces of said upright portions, and a gold brazing metal fusing saidWire laterals to said uprights and said half sections being intimatelyjoined together along the conjoint edges in a plane coincident to theaxis of said frame.

6. The method of fabricating a hollow grid structure for electronicdischarge devices including a rectangular frame member of similar halfsections having arcuate upright portions and outwardly bent transversearms connecting said portions, which comprises matching said sections inpairs with said arcuate portions in abutting relation to form a hollowstructure of elongated oval cross-section and said arms in parallelpairs at each end, winding a continuous wire uniformly around said framebetween said arms, temporarily fixing the ends of said winding to saidarms,

. applying a low melting point metal to said upright portions out ofengagement with said continuous wire, heating said frame assembly to asufficient temperature to melt said low melting point metal andflow saidmetal over the successlve turns of wire in contact with said uprightportions, and brazing the abutting edges of said in pairs with saidarcuate portions in abutting.

relation to form a hollow structure of elongated oval cross-section andsaid arms in parallel pairs at each end, winding a continuous helix offine wire in contact with said abutting upright portions and extendingin uniform pitch across the transverse arms from said fine wire, heatingsaid structure in a non-oxidizing atmosphere, fusing said turns of wireto flow by, capillary attraction along the contacting edges of saiduprights, re-

moving the cement from the ends of said wire,

10 attaching a few turns of gold wire around diagonally disposed ends ofsaid uprights removed from said fine tungsten wire, heating saidstructure in a hydrogen filled oven at a temperature of approximately1070 0., evaporating said cement, fusing said gold wire to flow bycapillary action along the abutting surfaces of said upright portions tobond said wire thereto and rigidly braze said half sections together.

EDWARD J. WALSH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,444,438 White Feb. 6, 19231,472,505 Trimble Oct. 30, 1923 1,842,176 Jones Jan. 19, 1932 1,859,678Nachumsohn May 24, 1932 1,893,466 Gowen Jan. 3, 1933 1,934,097 SimonNov. 7, 1933 2,004,246 Kershaw June 11, 1935 2,183,635 Barker 1 Dec. 19,1939 2,188,906 Lackey Feb. 6, 1940 2,197,753 Liebman Apr. 23, 19402,225,853 Baker et al. Dec. 24, 1940 2,255,906 Umbreit Sept. 16, 19412,279,831 Lempert Apr. 14, 1942 2,472,760 Ratchford June 7, 1949 FOREIGNPATENTS Number Country Date 7 477,787 Great Britain Jan. 6, 1938

